The invention relates generally to new ferroelectric chiral smectic liquid crystals, and more particularly to novel liquid crystals showing three stable states or tristable phase to be preferably used as a display element and above all for an electrooptical device owing to particularly high speed of response to an electric field.
Various electrooptical devices utilizing liquid crystals of nematic type have been developed and practically used in a dynamic scattering (DS) mode, twisted nematic (TN) mode, guest-host (G-H) mode and supertwisted nematic (STN) mode. Such devices are, however, not satisfactory due to the slow speed of response of the liquid crystals in the order of several m. sec.-several decades of m. sec. The reason why the response speed is slow is considered to lie in that torque to move liquid crystal molecules, which depends on the anisotropy of dielectric constant, is not so strong.
In order to solve such problem, ferroelectric liquid crystals were found and developed by Meyer et al (Le Journal de Physique, 36, 1975, 1-69), which have an spontaneous polarization (Ps) and consequently a higher value of the torque=Ps x E (impressed voltage), which results in the higher response time speed in the order of several .mu.sec. to several decades of .mu.sec. In JP-A63-307,837, improved ferroelectric bistable liquid crystals are disclosed.
Meanwhile, an electrooptical device utilizing liquid crystals of said ferroelectric liquid crystals have been proposed. For instance, JP-A Sho56-107,216 discloses a device in which two molecular orientations, which are made parallel to a wall surface by releasing the twisted structure thereof owing to the force of the wall surface, may be varied depending the polarity the impressed electric field. This is, however, theoretically based on the presence of a liquid crystal showing an ideal bistable phase as shown in wave shapes of response to the electric field as shown in FIG. 5.
In fact, however, such ideal compounds have not yet been found. The actually provided liquid crystals of bistable phase show the wave shapes of response as shown in FIG. 6. When using such liquid crystal e.g. in a photoswitching circuit, as the impressed voltage is changed from the .crclbar. side to the .sym. side, the transmission factor (%) is gradually changed so that the simple ON-OFF change of the impressed voltage cannot sufficiently attain the purpose.
Furthermore, so far as the liquid crystals of bistable phase having been synthesized until now are concerned, it can not realize a monodomain state, which is a perfect molecular orientation, in the Sc.sup.* state where there is no electric field, but results in a disorder of the molecular orientation which is called "twist". Thus, it is difficult to realize perfect two states orientation as referred to, above all in a larger area.
Furthermore, when dynamically driving, inevitably the contrast is lowered and the visual field angle is narrowed due to the low threashold of the voltage causing a change of brightness in a specific range. Since the liquid crystals of bistable phase having been synthesized until now show not the hysteresis as shown in FIG. 5 but that as shown in FIG. 6 so as not to have the memory effect. In order to hold the stable Sc.sup.* phase for the liquid crystals, it is necessary to continuously impress the voltage V.sub.3 in FIG. 6 or continuously apply a high frequency, either of which causes a considerably much amount of energy loss.